Wiki - HR-vs-Power

... added HR vs Power chart explanation to Wiki
Merge pull request #1008 from Joern-R/pull2

Wiki - HR-vs-Power
Don't blank CP chart in trends

.. when there is no ride selected / manual entry because
   we plot a date range not a single ride !
Small-Plot - wrong Axis Scaling and Labels

... wrong Axis IDs causing the following problems
... X-Axis - not considered time, but high (wrong label/wrong scale)
... Y-Axis - altitude curve not properly scaled
... use similar minimum scale for altitude like AllPlot
... attach Alt-curve first (to be in background)

(used/tested at HrPw window, Split Activity Wizard, Merge Activity
Wizard)
Merge pull request #1010 from Joern-R/pull3

Small-Plot - wrong Axis Scaling and Labels
Add Lactate Threshold section to Science Doc

.. inspired by Prof Seilers layman's document on Lactate Threshold.
   [may still be available here: https://www.dropbox.com/s/vgxjbo32i0o94nt/The%20Lactate%20Threshold_Seiler.pdf]
merge upstream into master